A broiler is a type of chicken raised specifically for meat production. Broilers comprise breeds that were specially bred for large scale, efficient meat production and growth in comparison to egg-producing breeds or traditional dual-purpose breeds. Broilers typically have very fast growth rates, a high feed conversion ratio, and low levels of activity, reaching harvest weight in about five weeks.
Annual broiler production in the United States has grown at a significant rate over the past four decades. In 2007, it was reported that the number of broiler pounds rose from 8 billion in 1966 to 49 billion in 2006. The increase in poultry production, particularly broilers, has been attributed to an increase in number of large-scale broiler houses and feeding operations. One measure states that in 2003 alone, over 32 billion broilers were produced by four companies in the United States, which companies run their own conglomerate of many very large scale industrial farms. Further, the U.S. broiler industry is mainly concentrated in the southeastern states, including Georgia, Arkansas, Alabama, Mississippi, and North Carolina, producing over 85% of the total pounds of broiler meat in the U.S.
Large commercial broiler operations generate enormous quantities of waste annually, and the management of this waste is largely unregulated. This waste, termed “broiler litter” in the industry, is composed of a mixture of bedding materials (straw or wood shavings), chicken excreta, feathers, and spilled feed. Its high organic carbon and nutrient contents along with its believed positive effects on soil have encouraged farmers to use it on agricultural soils. Many broiler producers simply “recycle” the waste by mixing with top soil in crop fields or the like. However, a single broiler bird generates approximately 1.5 kg of litter in a ten-week life cycle. The bulkiness of the waste generated and its high transportation costs have been a predicament to producers who would like to move the waste to surrounding counties and other states because they cannot use all of it on their own farms.
All told, a large majority of the domestic amount of broiler waste is applied directly to the land without any pretreatment. This can have an undesirable effect on the soil or water run off over time. In particular, broiler litter contains a high concentration of inorganic phosphorous (“P”) that originates from phytic acid and its phytate salts present in broiler feed. Broilers are typically fed corn and soybean blend diets fortified with vitamins and minerals. Corn seeds contain P mainly in phytate form. In this form, P is generally not bioavailable to broilers because they lack the digestive enzyme, phytase, required to separate P from the phytate molecule. Because phytate from these grains is unavailable for absorption, the unabsorbed phytate passes through the gastrointestinal tract of birds, thus elevating the P content of the manure. Continuous application of broiler litter to cropland or pastureland therefore may lead to accumulation of P in topsoil over time.
One recent investigation reported that over application of broiler litter results in increased concentrations of P in the soil solution and P enrichment of surface waters (see Toor et al., Advances in Agronomy, 89:1-72, 2006). In addition, when poultry waste is applied to crop field to provide nitrogen (“N”) requirements, this results in a net P balance in soil because more P is applied than is typically removed by harvested crops (see Sharpley, Poul. Sci., 78:660-673, 1999). Consequently, application of poultry litter can lead to a build-up of P in soils because manure N/P ratios are lower than crop N/P requirement ratios.
Run off of P from these over fertilized soils to nearby water bodies has also been established to cause eutrophication of lakes and streams (see Paudel et al., Waste Management, 25:1083-1088, 2005). In 2002, the US Environmental Protection Agency (USEPA) set permissible maximum total P concentrations for lakes, reservoirs, rivers, and streams. Although concentrated animal feeding operations have been regulated for more than 25 years, continued impairment of waters has led the Federal government to scrutinize such operations as potential sources of pollutants more scrupulously. Thus, there is a need to develop methods that would reduce P content of broiler litter before its application to agricultural land in order to avoid a long-term build-up of P in soil.
One of the approaches used to regulate available P content of broiler litter is the use of alum (Al2(SO4)3—aluminum sulfate). Alum has been used extensively in poultry houses to reduce P availability in the poultry litter. When mixed with broiler litter, aluminum sulfate reacts with moisture in the litter to reduce ammonia volatilization and tie up soluble P precipitate it as AlPO4, which then can be disposed of separately. While the use of alum has reduced available P, it has increased the total and water-soluble aluminum concentration in the litter (see Sims et al., J. Environ. Qual., 27:277-293, 2002). In addition, alum is relatively expensive and its use has had limited success at controlling soluble P levels.
Another approach utilized in the poultry industry is addition of the phytase enzyme to broiler diets before pelleting. Such phytase addition can result in a 15% to 25% decrease in total P in the litter compared with conventional diets. This approach, however, is also not ideal because the pelleting process usually denatures some of the phytase, making it less effective. Heat-stable enzymes able to withstand the pelleting process are being developed, but they are very expensive and cost prohibitive. The overall P decrease achieved may not be high enough to justify the cost.
Other areas that have encountered the problem of P removal include wastewater treatment and swine waste disposal. For wastewater treatment, chemical precipitation has been employed, which is a physicochemical process consisting of the addition of a divalent or trivalent cation salt to wastewater to cause precipitation of an insoluble metal-phosphate molecule that is thereafter settled out by sedimentation. The most suitable metals are iron and aluminum, added as chlorides or sulfates, and generally large scale chemical precipitation is performed using low-cost sorbents such as alum sludge, and red mud. Sometimes, the process is aided by addition of anionic polymers that assist in solid separation. In other instances, calcitic lime is used to precipitate calcium phosphate. Because P precipitation is pH dependent, however, such methods are not universally effective in providing high P removal rates. With respect to livestock waste in particular, Meers et al. (Water, Air, and Soil Pollution, 169:317-330, 2006) have suggested that the use of flocculants (e.g., FeCl3, FeCl2, FeClSO4, poly-aluminum chloride) followed by adsorption or precipitation of substrates can be efficient in reducing P levels of liquid pig manure. In the swine industry, P removal has also been achieved by the precipitation of P as a part of a mineral known as struvite, which also contains Mg and NH4+. This method reportedly has achieved reductions of 90% of soluble P concentrations in 140,000 liters (˜37, 000 gallons) of swine slurry. Others have tried to treat swine waste with chemical additives at dosage levels of 1,500 mgL−1 (5.4 mM Fe+3) of ferric chloride or aluminum sulfate, and reported successful removal of 86% and 76% of P, respectively (see Ndegwa et al., J. Envir. Engrg., 127:1111-1115, 2001). None, however, have approached the problem of how to successfully remove the P from solid poultry litter.
Thus, there remains a need in the art for methods to reduce the amount of P contained in poultry litter before its application to land.